You can label your contacts as "Customer" "Lead" or "Other" by using the "Contact type" drop down. Easily see your Customers vs Leads with the Default Customer List and Default Lead list.
Click "Contacts" from the left navigation
You will have two contact lists by default that correspond with the Contact Type field.
The first list is titled "Customers" and contains all contacts carrying a contact type of "Customer".
The second list is titled "New Leads" and is populated with any contacts created in the last 30 days that carry a contact type of "Lead".
Add a contact
Click on "Add a contact" from the contacts page
Select the Contact Type from the drop down
Click Save and exit